Since you are jailbroken with jailbreakme.com you will also need to download and use iReb to prepare the iphone for a custom firmware. Otherwise you may get a error in itunes when you try and restore to the custom firmware. And yes this will preserve your baseband so that after you finish the restore to the custom pre-jailbroken firmware all you have to do is go into cydia and install ultrasn0w and you will be unlocked again.